1. Field of the Invention
This invention relates to electroacoustic apparatus, and more particularly, to surface acoustic wave reflection mode devices.
2. Description of the Prior Art
The use of acoustic reflective array compressors (RAC) for pulse compression and expansion, as well as for filters, is well known in the electrical arts. A typical RAC implementation consists of dispersive reflective grooves or gratings oriented to reflect an acoustic beam launched by an input transducer to an output transducer. The theory and operation of such an implementation is provided in an article entitled "Design of Reflective-Array Surface Wave Devices" published on pages 177-198 of Wave Electronics 2, July 1976, by Melngailis, et al. One technique used to fabricate the grooved reflective array is an ion beam etching process such as described by Martin in U.S. Pat. No. 3,882,429. Proper pulse amplitude equalization or weighting in the frequency domain can be achieved by adjusting the depth of the reflective grooves by ion beam etching, for example, to compensate for energy loss as the acoustic pulse propagates through the array.
In an article entitled "Amplitude Weighting of SAW Reflecting Array-Structures" published on page 390 of the "Proceeding of IEEE Ultrasonics Symposium", 1974, F. G. Marshall, et al describes another method of pulse weighting or equalization in which a form of amplitude weighting called chevron weighting is achieved by employing metallic fingers that can be fabricated by using a single step photo lithographic process instead of the more difficult ion beam etching process. Amplitude weighting is accomplished by progressively scattering more of the acoustic beam out of the transmission path. However, this chevron weighting scheme reflects only part of the aperture of the acoustic wave which introduces phase distortion which in turn limits or reduces the degree of frequency side lobe suppression of the output signal possible for a given RAC. This limiting of the side lobe suppression has deleterious effects in that it reduces the information content of the reflected wave.
It is therefore desirable to provide an acoustic reflective array device which provides pulse amplitude equalization or weighting in the frequency domain without introducing phase distortion of the wave.